What effect does rising DO levels above 0.5 mg/L have in an anoxic zone?

In an anoxic zone, the biological process known as denitrification occurs, in which nitrate (NO3-) is reduced to nitrogen gas (N2) in the absence of oxygen. Denitrifying bacteria thrive in environments where oxygen is limited, as they use nitrate or nitrite as an electron acceptor instead of oxygen, which is not available in significant amounts.

When dissolved oxygen (DO) levels rise above 0.5 mg/L in an anoxic environment, it can interfere with the denitrification process because these bacteria prefer conditions where oxygen is absent. This shift can lead to a scenario where nitrifying bacteria, which oxidize ammonia (NH3) to nitrate (NO3-), could become more active. Although nitrification is typically more favorable in aerobic conditions, the presence of even low levels of dissolved oxygen can initiate a shift in microbial populations and their metabolic activities.

Therefore, the correct interpretation is that rising DO levels can promote nitrification by allowing some nitrifying bacteria to become more active. This shift is significant in managing wastewater processes, as controlling DO levels is crucial for optimizing the different stages of nitrogen removal in treatment systems.